The present invention relates to drilling fluids which contain non-aqueous solvents such as, e.g. N-methyl-2-pyrrolidone.
During the drilling of an oil well, a usually aqueous fluid is injected into the well through the drill pipe and recirculated to the surface in the annular area between the well-bore wall and the drill string. The functions of the drilling fluid include: lubrication of the drill bit, transportation of cuttings to the surface, overbalancing formation pressure to prevent an influx of oil, gas or water into the well, maintenance of hold stability until casings can be set, suspension of solids when the fluid is not being circulated, and minimizing fluid loss into and possible associated damage/instability to the formation through which drilling is taking place.
Proper balancing of formation pressure is obtained by establishing fluid density at the desired level usually via the addition of barite (greater than or equal to 95% barium sulfate). Transportation of cuttings and their suspension when the fluid is not circulating is related to the fluid viscosity and thixotropy which depend on solids content and/or use of a polymer. Filter loss control is obtained also by the use of clays and/or added polymers.
Fluid properties are constantly monitored during the drilling operations and tailored to accommodate the nature of the formation stratum being encountered at the time. When drilling reaches the producing formation special concern is exercised. Preferentially low solids content fluids are used to minimize possible productivity loss by solids plugging. Proper fluid density for overbalancing formation pressure may be obtained by using high salt concentration aqueous brines while viscosity and filter loss control may be obtained by polymer addition. Substantial future oil well drilling will be at depths between 15 and 30 thousand feet where temperatures encountered can be 350.degree. F. Temperatures such as these, coupled with the desire for low solids content and preferably minimum added solids, require brine tolerant and high temperature stable polymers for viscosity and filtration control. Conventionally employed polymers such as starch, carboxymethyl cellulose, and modified polyacrylates are not stable at such high temperatures and some have severe brine tolerance limitations.
Current high density, clear brine systems utilize hydroxyethyl cellulose polymers and related materials as viscosifiers, but these are normally unstable at about 150.degree. C., and tend to crosslink and gel with time and temperature which may cause various drilling operational problems.